The present invention relates to techniques for generating an analogue signal based on a one-bit signal, and more particularly to a technique which is designed to suppress transitory noise occurring when a shift is made to a silent state at the time of turning on/off of a power supply, at the time of resetting or the like or when the silent state ends and which is applicable to apparatus, such as electronic musical instruments, for generating an audio waveform signal, digital-to-analog converters, etc.
Today, one-bit digital-to-analog (DA) converters are used extensively to convert a one-bit digital audio signal, generated or processed in accordance with PWM (Pulse Width Modulation), PDM (Pulse Density Modulation) or the like, into an analogue audio signal and output the converted analogue audio signal. Amplitude value of a raw waveform in a one-bit audio signal input to such a one-bit DA converter is represented by density of pulses, the one-bit DA converter generates an analog signal by filtering the input one-bit audio signal by means of a lowpass filter (LPF). In recent years, there has been known a technique which, in generating a one-bit digital signal from a raw waveform signal by use of a ΔΣ modulator, oversamples a digital audio signal (raw waveform) at a high sampling frequency to cause quantization noise to distribute over wide frequency bands, then performs noise shaping, by means of the ΔΣ modulator, for shaping the quantization noise such that the level of the quantization noise decreases in a low frequency band and increase in a high frequency band to thereby reduce the noise level in an audible frequency range (see, for example, Japanese Patent Application Laid-open Publication No. HEI-11-122112).
Also known is a DA conversion technique which generates a single analogue signal by use of two-phase one-bit signals consisting of positive-phase and reverse-phase signals (i.e., two differential signals or balanced signals) (see, for example, Japanese Patent Application Laid-open Publication No. 2006-80685). More specifically, according to this technique, the above-mentioned ΔΣ modulator is applied to a one-bit switching amplifier to generate two-phase differential signals from a one-bit digital signal that is an output of the ΔΣ modulator, and then an analog signal is generated from the differential signals by means of a power switch and an LPF. It has also been conventionally known to implement the ΔΣ modulator by a full differential circuit.
A one-bit signal in the aforementioned one-bit DA converter represents peak values of an original waveform by density of a pulse train, and it is a signal where, in a silent state, timing to output a pulse and timing to not output a pulse occurs alternately at an equal ratio (i.e., with a duty cycle of 50%). Normally, as a power supply to an electronic musical instrument, audio equipment or other apparatus is turned on from an off state, energization of the apparatus is started from a state where the apparatus is not energized at all. At that time, operation of a DA conversion system provided in the apparatus is initially started in a silent state. Thus, when shifting to the silent state in response to the turning-on of the power supply (i.e., power supply ON), the DA conversion system suddenly shifts from the non-energized state to a state in which there is output a one-bit signal where timing to output a pulse and timing to not output a pulse occurs alternately at an equal ratio, so that transitory noise would undesirably occur. When the DA conversion system shuts down to the power supply OFF state too, there would sometimes occur the problem of transitory noise. Namely, when the silent state ends in response to the power supply OFF, the one-bit signal where the timing to output a pulse and the timing to not output a pulse has so far occurred alternately at an equal ratio is suddenly shut down, so that transitory noise would arise.